1. Field of the Invention
The present invention relates to an antenna arrangement for providing a nearly frequency independent field distribution using a small feedhorn and, more particularly, to an antenna arrangement which uses a main reflector combined with a feed arrangement comprising an imaging ellipsoid subreflector, confocally disposed at a first focus with the main reflector, and a small feedhorn disposed such that both the apex of a spherical wavefront launched therefrom is positioned at the other focus of the subreflector and the surface and boundary of the aperture of the feedhorn are substantially at the image surface and boundary, respectively, of the main reflector.
2. Description of the Prior Art
When the far-field of a horn is used to illuminate the aperture of a paraboloid, the resulting distribution varies with frequency. This variation, which is due to the phase-center and beam-width frequency-dependence of the horn has been an important limitation in the past and has been described in, for example, the article "An Improved Antenna For Microwave Radio System Consisting of Two Cylindrical Reflectors and a Corrugated Horn" by C. Dragone in BSTJ, Vol. 53, No. 7, September 1974 at pages 1351-1377. A simple way to realize a frequency independent illumination over a large aperture is to use a very long horn so as to guide a spherical wavefront from the apex of the horn to the antenna aperture. In this regard see, for example, U.S. Pat. No. 3,936,837 issued to H. P. Coleman et al on Feb. 3, 1976 and U.S. Pat. No. 3,949,404 issued to J. C. Fletcher et al on Apr. 6, 1976. Such long horns, however, are expensive and heavy.
The problem remaining in the prior art is to provide an antenna arrangement which has a nearly frequency independent field distribution over a large antenna aperture with minimal spill-over which doesn't use long feedhorns.